The problems associated with the lubrication of automotive rear axle and transmission gears are well known to those of skill in the art. Such gears are usually hypoid in nature and for reasons inherent in their design develop great pressures between the contacting metal surfaces of the gear. These pressures literally squeeze out ordinary mineral oil lubricants from between the gears and result in metal-to-metal contact between the parts of the gear under extremely high pressures. Such metal-to-metal contact soon leads to deformation and destruction of the gear surfaces.
To alleviate these and other problems it has become a common practice to add to gear oils certain chemical substances which are generally called additives. Among such additives are extreme pressure agents which are widely believed to react with the metal surfaces of the gear under the elevated temperatures and high pressures generally encountered during operation of the gear to form thin films. These newly formed layers act as separants and lubricants for the gear surfaces and thus prevent or reduce destructive metal-to-metal contact.
Recently a demand has arisen for lubricants which will function satisfactorily as gear lubricants under a wide range of temperature conditions. Such compositions are characterized by relatively small changes in their viscosities with changing temperature and they are commonly graded according to SAE standards according to their viscosities at low (e.g., 0.degree. F.) and high temperatures (e.g., 210.degree. F.) as is explained in further detail below. Because of this grading, such gear oils are often said to be "multigraded." In terms of widely accepted concepts such multigraded gear lubricants can also be said to have high viscosity indices. Multigraded gear lubricants thus have the desirable property of being able to function immediately, though cold, upon being put into service and continue to function satisfactorily as they become heated during operation.
In addition to being multigraded, it is desirable that lubricant compositions especially adapted for use as gear lubricants, exhibit shear stability. This means that they will not degrade or lose their viscosity as a result of the shearing forces encountered during their use. Compositions exhibiting desirable shear stability will be found to generally have a viscosity within 85 to 95% of their original viscosity after many hours (say 1000) of service. It has been recognized that many ordinary viscosity index improvers commonly added to crankcase lubricating oils, such as high molecular weight polyisobutylenes and polyacrylates, do not possess the desired shear stability for use in improving the viscosity properties of gear lubricants.
It has now been found that, in accordance with this invention, multigrade lubricants exhibiting exceptional shear stabilities can be made by combining in certain proportions base oils, polymers of specified structure and molecular weight and extreme pressure agents.